WASHINGTON—An outside political group supporting House Democrats is casting blame for the government shutdown on nine House Republicans in a series of television and online ads.

House Majority PAC is launching a six-figure broadcast television ad buy this week against Republican Reps. Steve Southerland of Florida, Joe Heck of Nevada and Dave Joyce of Ohio. The ads accuse the Republicans of causing the federal government shutdown. The GOP seats are top priorities for Democrats in the 2014 midterm elections.

The group is also planning online ads against House Republicans representing districts in California, Colorado, Illinois, Pennsylvania, New York and Minnesota.

The TV ads will air in Tallahassee, Fla., Las Vegas, and Cleveland, part of a larger effort by both parties to gain leverage in the government shutdown.